Resistor in an integrated circuit

ABSTRACT

A resistive element having two vertical resistive portions placed in two holes formed in the upper portion of a substrate and a horizontal resistive portion placed in a buried cavity connecting the bottoms of the holes.

FIELD OF THE INVENTION

The present invention generally relates to a resistor, and morespecifically to a resistor in an integrated circuit.

DISCUSSION OF THE RELATED ART

An example of a resistor currently used in integrated circuits is aresistor having the shape of a polysilicon or metal (TaN) strip placedabove a silicon wafer and insulated therefrom by an insulating layer.

Another example of a known resistor is a P- or N-type doped siliconportion formed in the upper portion of a silicon wafer.

A disadvantage of such resistors is that the wafer surface areas neededcan be very large.

SUMMARY OF THE INVENTION

An object of at least one embodiment of the present invention is toprovide a resistor structure taking up a small surface area of a siliconwafer.

At least one embodiment of the present invention further provides amethod for manufacturing such a resistor.

To achieve these and other objects, at least one embodiment of thepresent invention provides a resistive element comprising two verticalresistive portions placed in two holes formed in the upper portion of asubstrate and a horizontal resistive portion placed in a buried cavityconnecting the bottoms of the holes.

At least one embodiment of the present invention further provides aresistor comprising several resistive elements such as those describedhereabove connected to one another by resistive strips placed on thesubstrate.

According to one embodiment of the resistive element, the two holesconnected by the buried cavity form a conduit, the vertical andhorizontal resistive portions being formed by an insulated resistivelayer covering the conduit walls.

According to another embodiment of the resistive element, the substrateis the silicon wafer, said resistive layer being separated from thesubstrate by an insulating layer such as a silicon oxide layer.

According to another embodiment of the resistive element, the conduit isfilled with a filling material such as polysilicon.

According to another embodiment of the resistive element, the resistivelayer and the filling material are separated by an insulating layer suchas a silicon oxide layer.

According to another embodiment of the resistive element, the resistivelayer is formed of polysilicon or of a metal.

According to another embodiment of the present invention furtherprovides a method for forming a resistive element in a substratecomprising the steps of: forming, by anisotropic etch, two holes in theupper portion of a substrate; forming, by isotropic etch at the bottomof the holes, a cavity connecting the bottom of the two holes, the holesand the cavity forming a conduit; and performing a conformal depositionof a resistive layer against the conduit walls.

According to an embodiment of the above-mentioned method, the methodcomprises, prior to the conformal deposition of the resistive layer, astep of conformal deposition of a first insulating layer, and furthercomprises a step of conformal deposition of a second insulating layercovering said resistive layer, as well as a step of filling of theconduit with a filling material such as polysilicon.

According to an embodiment of the above-mentioned method, on forming ofthe resistive layer against the walls of the conduit, a resistive layerforms at the substrate surface, the method further comprising a step ofetching the resistive layer at the substrate surface to form resistivestrips.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of the presentinvention will be discussed in detail in the following non-limitingdescription of specific embodiments in connection with the accompanyingdrawings.

FIGS. 1A, 2, 3, 4A, and 5 are cross-section views and FIGS. 1B and 4Bare top views of structures obtained in successive steps of a method forforming a resistor according to the present invention;

FIG. 6 is a cross-section view of another example of a resistor obtainedaccording to a variation of the method described in relation with FIGS.1 to 5; and

FIG. 7 is a top view of an example of a resistor according to anembodiment of the present invention.

DETAILED DESCRIPTION

For clarity, the same elements have been referred to with the samereference numerals in the different drawings and, further, as usual inthe representation of semiconductor components, the various drawings arenot to scale.

A resistor according to the present invention may be called athree-dimensional, “3D,” resistor. The resistor is formed of an assemblyof elementary resistive elements formed in the upper portion of asubstrate, such as a silicon wafer. A resistive element comprises two“vertical” resistive portions, placed in two holes formed in the uppersubstrate portion and a small “horizontal” resistive portion placed in aburied cavity connecting the bottoms of the two holes.

A method for forming such a resistor is described hereafter in relationwith FIGS. 1 to 6.

In an initial step, illustrated in FIGS. 1A and 1B, an anisotropic etchof a substrate 1 is performed to form pairs of holes 2 a/2 b and 3 a/3 bin the upper portion of substrate 1. Substrate 1 for example is asilicon wafer. The etching may be performed according to a deep reactiveion etching (DRIE). Substrate 1 is biased so that the substrate etchingby ionized gas molecules is performed “vertically”. The used gas mixturemay comprise a “passivating” gas which reacts with the substrate to forma thin insulating layer. A mixture of an “etchant” gas such as SF₆ and apassivating gas such as C₄F₈ is for example used. When a gas mixturecomprising a passivating gas is used, a thin insulating layer forms onthe hole walls along their forming.

An isotropic etch of substrate 1 is then performed, at the bottom ofholes 2 a/2 b and 3 a/3 b, to form “buried” cavities at the bottom ofeach of the holes. The two holes of each pair are placed close enough toeach other for the buried cavities formed at the bottom of each of theholes to connect to form a single buried cavity. Thus, holes 2 a and 2 bare connected by a buried cavity 5 and holes 3 a and 3 b are connectedby a buried cavity 6. This isotropic etch may be performed according toa reactive ion etching substantially identical to that used to form theholes, except that substrate 1 is no longer biased and that the amountof passivating gas may be smaller.

As visible in FIG. 1B, holes 2 a, 2 b, 3 a, 3 b have in this example asubstantially cylindrical shape. Buried cavities 5 and 6, having asubstantially oval shape in top view, are shown in dotted lines aroundhole pairs 2 a/2 b and 3 a/3 b.

It is considered hereafter that holes 2 a, 2 b and buried cavity 5 forma conduit 10. Similarly, holes 3 a, 3 b and buried cavity 6 form aconduit 11.

At the next step, illustrated in FIG. 2, several conformal depositionsof a thin insulating layer 20, of a resistive layer 21, and of aninsulating layer 22 are successively deposited on thepreviously-obtained structure. These three thin superposed layers coverthe walls of conduits 10 and 11 as well as the upper surface ofsubstrate 1.

Insulating layers 20 and 22 may be obtained by a conventional thermaloxidation method or by a low-pressure chemical vapor deposition, orLPCVD. Insulating layer 20 and 22 are for example formed of siliconoxide.

Resistive layer 21 may be formed of polysilicon, doped or undoped, or ofa metal such as tantalum nitride. Such resistive layers may be depositedaccording to an LPCDV method or according to an atomic layer chemicalvapor deposition, or ALCVD.

At the next step, illustrated in FIG. 3, conduits 10 and 11 are filledwith a filling material 30 such as polysilicon. In this example, fillingmaterial 30 also covers the surface of substrate 1.

At the next step, illustrated in FIGS. 4A and 4B, filling material 30,insulating layer 22, and resistive layer 21 are successively etched toexpose the surface of insulating layer 20, while keeping strips A, B,and C of these various materials. The strips are thus electricallyinsulated from one another. Each strip A, B, and C is formed of astacking of a portion A21, B21, C21 of resistive layer 21, of a portionA22, B22, C22 of insulating layer 22, and of a portion A30, B30, C30 offilling material 30. The ends of central strip B cover holes 2 b and 3 aof conduits 10 and 11. One end of outer strip A covers hole 2 a ofconduit 10 and one end of outer strip B covers hole 3 b of conduit 11.Strips A, B, and C are in this example aligned in top view.

At the next step, illustrated in FIG. 5, outer strips A and C arepartially etched on the side opposite to holes 2 a and 3 b. Fillingmaterial 30 and insulating layer 21 are successively etched to enableaccess to the ends of resistive portions A21 and C21 of strips A and C.

The resistor shown in FIG. 5 comprises two resistive elements R1 and R2respectively formed in conduits 10 and 11. The substantially cylindricalportions of resistive layer 21 placed in holes 2 a/2 b and 3 a/3 b formvertical resistive portions R1 a/R1 b and R2 a/R2 b. The oblong portionsof resistive layer 21 placed in buried cavities 5 and 6 form“horizontal” resistive portions R1 c and R2 c.

It should be noted that in the case where substrate 1 is formed of aninsulating material such as glass, insulating layer 20 is not necessary.

FIG. 6 illustrates a resistor obtained according to a variation of theabove-described method and more specifically according to an alternativeembodiment of the initial etch steps implemented to form conduits 10 and11. In this example, the substrate used is an SOI-type(silicon-on-insulator) wafer, comprising a thick silicon layer 50covered with a thin insulating layer 51, itself covered with a siliconlayer 52. The forming of the conduits comprises, in this embodiment, theetching of holes across the entire thickness of silicon layer 52according to an anisotropic etch method, followed by the extension ofthis same etching, once the holes have been formed, to form buriedcavities of connection between the bottom of the holes. The etching ofthe buried cavities is performed by favoring a normally parasiticphenomenon, of lateral “ricochet” etch on thin insulating layer 51, thisphenomenon being known as the “notching”.

FIG. 7 is a top view of an example of a resistor according to anembodiment of the present invention comprising an assembly of resistiveelements formed in the upper portion of a substrate, such as thosedescribed hereabove. The resistive elements are connected to one anotherby conductive strips placed on the substrate. The conductive strips areshown in full lines, the hole openings are shown by circles in dottedlines placed under the ends of the conductive strips, and the buriedcavities are shown by ovals in dotted lines surrounding two holeopenings.

The resistive elements are arranged in rows L1 to L6 which comprise 5resistive elements each, row L1 being shown at the bottom of thedrawing. The resistive elements of a same row are aligned, that is, thehole openings and the buried cavities in which the resistive elementsare formed are aligned with respect to one another. Similarly, theconductor strips interconnecting resistive elements of a same row arealigned and have a substantially rectangular shape. The elements of arow are connected to those of a neighboring row by a resistiveconnection strip, U-shaped in this example. Three resistive connectionstrips connect rows L1/L2, L3/L4, and L5/L6 to the left thereof and tworesistive connection strips connect rows L2/L3 and L4/L5 to the rightthereof. The resistor thus has in top view the shape of a serpentine.The right ends of the rightmost resistive strips of rows L1 and L6 arethe ends of the serpentine and form contact pads P1 and P2 of theresistor.

As a non-limiting indication, the resistor shown in FIG. 7 is formed ofresistive elements, each exhibiting the following features:

hole diameter: 1 μm;

hole depth: 50 μm;

interval between holes: 2 μm;

maximum diameter of the buried cavity: 3.5 μm;

maximum width of the resistive strips: 2 μm; and

the resistive elements are placed with respect to one another asfollows:

interval between the holes of resistive elements of neighboring rows: 4μm;

interval between two neighboring holes of two successive resistiveelements of a same row: 4 μm.

It should further be noted that for a given resistance value, thesurface area taken up by a resistor according to an embodiment of thepresent invention is much smaller, from 5 to 10 times as small, as thattaken up by a conventional resistor formed at the surface of a siliconwafer.

Of course, the present invention is likely to have various alterations,modifications, and improvements which will readily occur to thoseskilled in the art. In particular, those skilled in the art can devisevarious forms of resistors formed of differently-arranged elementaryresistive elements.

Further, those skilled in the art can devise other methods for forming aresistive element according to the present invention. Previously-formedinsulated conduits may for example be filled with a resistive materialsuch as polysilicon.

Such alterations, modifications, and improvements are intended to bewithin the spirit and scope of the invention. Accordingly, the foregoingdescription is by way of example only and is not intended as limiting.The invention is limited only as defined in the following claims and theequivalents thereto.

1. A resistive element comprising two vertical resistive portionsdisposed in two holes formed in the upper portion of a substrate and ahorizontal resistive portion disposed in a buried cavity connecting thebottoms of the holes, the vertical and horizontal resistive portionscomprising an insulated resistive layer covering the walls of the holesand of the buried cavity.
 2. A resistor comprising several resistiveelements according to claim 1 connected to one another by resistivestrips placed on the substrate and formed from portions of saidresistive layer.
 3. The resistive element of claim 1, wherein thesubstrate is a silicon wafer, said resistive layer being separated fromthe substrate by an insulating layer.
 4. The resistive element of claim3, wherein the insulating layer comprises silicon oxide.
 5. Theresistive element of claim 1, wherein the holes and buried cavity arefilled with a filling material.
 6. The resistive element of claim 5,wherein the resistive layer and the filling material are separated by aninsulating layer.
 7. The resistive element of claim 5, wherein thefilling material comprises polysilicon.
 8. The resistive element ofclaim 5, wherein the insulating layer separating the resistive layer andthe filling material comprises silicon oxide.
 9. The resistive elementof claim 1, wherein the resistive layer is formed of polysilicon or of ametal.
 10. The resistive element of claim 1, wherein the resistive layerwas deposited in one processing step.
 11. The resistive element of claim1, wherein the resistive layer comprises a conformal deposition of aresistive material.
 12. The resistive element of claim 1, wherein theresistive layer was deposited by low-pressure chemical vapor depositionor atomic layer chemical vapor deposition.
 13. The resistive element ofclaim 1, wherein the resistive layer comprises tantalum nitride.
 14. Theresistive element of claim 1, wherein the substrate comprisessilicon-on-insulator.
 15. The resistive element of claim 1, wherein thediameter of the holes is about one micron.
 16. The resistive element ofclaim 1, wherein the depth of the holes is about 50 microns.